High temperature corrosive resistant cobalt-base alloys



United States Patent 3,549,356 HIGH TEMPERATURE CORROSIVE RESISTANTCOBALT-BASE ALLOYS Chester T. Sims and Adrian M. Beltran, Ballston Lake,

N.Y., assignors to General Electric Company, a corporation of New YorkNo Drawing. Filed Jan. 6, 1969, Ser. No. 789,390

Int. Cl. C22c 19/00 U.S. Cl. 75--171 2 Claims ABSTRACT OF THE DISCLOSURECobalt-base alloys having improved high temperature oxidation andcorrosion resistance consist essentially of, in percent by weight,carbon 0.1 to 0.8, nickel 8.5 to 11.5, chromium 24 to 35, tungsten 6 to9, tantalum 1 to 5, manganese up to 1 max., silicon up to 0.25 max.,boron 0.005 to 0.5, yttrium 0.01 to 1.0, iron up to 2.0 max., hafnium0.05 to 2.0, with the remainder essentially cobalt except forimpurities.

This invention relates to new and useful alloys. More particularly, itrelates to alloys which are characterized by good high-temperaturestrength and at the same time are resistant to oxidation and corrosionby combustion gases at elevated temperatures.

Equipment depending upon the driving force of combustion gases, as ingas turbines, operates more efficiently and with greater output as thetemperature rises. It is also well known that at elevated temperatures,which enhance this increased efliciency and output, the strength of manyalloys often decreases rapidly and they become more and more susceptibleto oxidation and corrosion caused by constituents in the combustion gasstream. Among such corrosive constituents are sulfur, sodium andvanadium.

As the operating temperature of such equipment rises, relatively smallimprovements in corrosion resistance and strength of the structuralmaterials become important. For example, in gas turbines operating ataverage temperatures of the order of about 1600 F., with peaktemperatures of about 2000 R, an improvement of only about 100 F. in theoxidation and corrosion resistance of the structural materialsrepresents a notable advance. To illustrate, an increase in operatingtemperature of a typical gas turbine from about 1500 F. to 1600 F.produces an increase in power output of about 14% and an increase inefiiciency of from about 1 to It is therefore a principal object of thisinvention to provide new and useful alloys which will permit substantialincreases in the operating temperatures of such equipment as gasturbines, while at the same time providing suitable strengthcharacteristics at such higher temperatures which can range up to 1900F., 2000 F. or higher. Another object of the invention is to provideimproved materials of construction for high temperature equipment ingeneral, which are subjected to corrosive atmospheres such as furnacesand the like.

Briefly, there are provided by the present invention stronghigh-temperature corrosion-resistant cobalt base alloys having a percentby weight content of carbon 0.1 to 0.8, nickel 8.5 to 11.5, chromium 24to 35, tungsten 6 to 9, tantalum 1 to 5, manganese up to 1 max., siliconup to 0.25 max., boron 0.005 to 0.5, yttrium 0.01 to 1.0, iron up to 2.0max., hafnium 0.05 to 2.0, with the remainder essentially cobalt, exceptfor incidental impurities such as phosphorus and sulfur which arepreferably held below a maximum of about 0.015%.

It has been found that alloys of the above precisely balanced metalliccomposition are characterized by substantial improvement in corrosionresistance at elevated temperature, while at the same time retainingsuitable 3,549,356 Patented Dec. 22, 1970 strength, ductility, and otherphysical characteristics for elevated temperature operation. Thematerials are also particularly useful in that they are particularlyadapted to precision investment casting techniques permitting theformation of various shaped structures suitable for high-temgeratureapparatus such as in the hot stages of gas turmes.

Those features of the invention which are believed to be patentable areset forth with particularity in the claims appended hereto. Theinvention will, however, be better understood and further advantages andobjects thereof appreciated from a consideration of the followingdescrip tion.

With regard to the present compositions, the relatively high range ofchromium, specifically from about 24 to 35 weight percent, is directlycontrary to prior art teaching which states that cobalt-base alloyshaving a chromium content of over about 25% by weight exhibit increasedscaling or deterioration due to corrosive influences at elevatedtemperatures. This prior art teaching is set forth, for example, inJournal of the Electrochemical Society, volume 103, No. 8, by Pfalnikaret al., entitled, High Temperature Scaling of Cobalt-Chromium Alloys.

As pointed out above, the present compositions comprise a carefullyformulated combination of constituents, each of which contributes to thedesirable characteristics obtained. Deviations in the proportions of thematerials destroy this critical balance, and result in materials whichhave been found to be wanting in essential characteristics. For example,when the carbon content is lowered beyond that prescribed, anundesirable loss of strength results. On the other hand, increasing thecarbon content above that stated would detract from the ruptureductility of the material and may also detract from weldability.Reduction of the chromium content below that set forth results indetrimental loss of oxidation resistance. The nickel,

iron and tungsten additives do not appear to be particularly criticalfor oxidation and corrosion resistance but it has been found desirableto have them present in the stated ranges for suitable structuralmechanical properties. Boron imparts ductility to the alloy butincreasing the boron beyond the content set forth causes detrimentallow-melting phases to form in the alloy. Yttrium, when used in loweramounts than that set forth above, results in decreased oxidation andcorrosion resistance. As a practical matter, amounts of yttrium greaterthan the upper prescribed limit are diflicult to retain effectively inthe product by normal melting procedures. Within the prescribed limits,yttrium greatly improves the scale spalling resistance of the alloyunder the thermal cycling conditions encountered in gas turbines, forinstance. Hafnium contributes strength to the alloy in the amountsindicated by modifying the type of carbides present in favor of carbideswith greater thermal stability. Lower amounts detract from thisfavorable characteristic while larger amounts than those indicated donot contribute any additional benefit.

The following example will illustrate the practice of the invention, itbeing realized that it is exemplary only and not to be taken as limitingin any way. There was prepared an alloy consisting of by weight percentcarbon 0.40, nickel 10.7, chromium 28.7, tungsten 7.4, tantalum 3.1,manganese 0.30, silicon 0.21, boron 0.016, yttrium 0.18, iron 0.26 andhafnium 0.15, with the remainder essentially cobalt except for sulfurand phosphorus within the abovementioned limit. This alloy wasinvestment cast and standard cast-to-size 0.252" diameter rupture barsand corrosion pins A3" diameter x 1%" long prepared.

Shown in Table I are the room temperature and high temperaturemechanical properties of the above exemplary alloy as compared with atypical prior art alloy,

specifically WI-52. The Larsen-Miller parameter (constant=2-0) is a wellknown numerical tool which combines time and temperature to allowcomparison of the capabilities of these alloys on a normalized basis.

TABLE II Mils per side Temp, Time, Surface Max. pene- Alloy rs. losstration TABLE I [A.Room temperature tensile test] Example Ultimate 0.2 70.027

tensile yield. yielt l Percent NI-52 i 888 g strength, strength,strength, Percent reduction 1 1 Alloy K s.i. K s.i. K s.i. elongation inarea a p e 8 5 0 From the above table it will be quite evident that the""f-' present alloy has far and away superior corrosion resist-[B.Stress-Rupture tests] Larsen- Percent Miller Temp., Stress, Life,Percent reduction parameter Alloy F. K s.i. hours elongation in area(0:20)

Example 1, 600 20 405. 5 18. 2 48. 3 4 6 1, 700 226. 4 21. 4 57. 0 48. 31, 825 10 50. 9 1, 900 6 54. 2

AV WI-52 1, 600 46. 3 g 1, 700 15 48. 7 1, 825 10 51. 2 1, 900 6 53. 7

From the above table it will be seen that the rupture strength isindicative of the present alloy using room temperature tensile strengthcomparable to the prior art alloy; the present material, however, hasabout three times the ductility as measured by reduction in areas. Thestress-rupture test results in Table I indicate that the stress-rupturestrength of the present alloy using the Larsen-Miller parameterindicated has a strength comparable to the prior art alloy at highstresses, with a clear crossover at low stresses in favor of the presentalloy. It will thus be seen that the present alloys are particularlyadaptable to applications such as stationary nozzle guide vanes orpartitions and similar applications which are characterized by highoperating temperatures and relatively low stresses.

Shown in Table II is the hot-corrosion resistance of the presentexemplary alloy as compared to the above prior art alloy. In this test,pin-shaped test pieces of the material of the above example and theprior art material were placed in the combustion gas stream flow in asimulated gas turbine burner apparatus at the 1900" F. indicatedtemperature using combustion Diesel oil (MIL-F- 16884) containing 1% byWeight sulfur at an air to fuel weight ratio of :1. Five ppm. sea salt(ASTM, D665-60) was continuously atomized into the chamber to simulate amarine environment, comparable to contamination by salt water duringservice. This produces a sulfidizing and oxidizing environment throughgeneration of sodium sulphate. The specimens were thermal cycled everyfifty hours to simulate gas turbine shutdown, this procedure beingproperly rigorous as it evaluates adherence properties of the protectivescale. After the times indicated, the surface loss and maximumpenetration were measured for each sample in terms of mils per side.

ance at elevated temperatures to the prior art alloy. Such corrosionresistance combined with the favorable strength characteristics atelevated temperatures makes these alloys very useful under lowstress-high temperature corrosive conditions which are experienced ingas turbines, furnaces and similar apparatus.

What we claim as new and desire to secure by Letters Patent of theUnited States is:

1. A cobalt base alloy characterized by good high temperature oxidationand corrosion resistance consisting essentially of about, by weight,carbon 0.1 to 0.8 percent, nickel 8.5 to 11.5 percent, chromium 24 to 35percent, tungsten 6 to 9 percent, tantalum 1 to 5 percent, manganese upto 1 percent max., silicon up to 0.25 percent max., boron 0.005 to 0.5percent, yttrium 0.01 to 1.0 percent, iron up to 2.0 percent max.,hafnium 0.05 to 2.0 percent, with the remainder essentially cobaltexcept for impurities.

2. A cobalt base alloy characterized by good high temperature oxidationand corrosion resistance consisting essentially of about, by weight,carbon 0.40 percent, nickel 10.7 percent, chromium 28.7 percent,tungsten 7.4 percent, tantalum 3.1 percent, manganese 0.30 percent,silicon 0.21 percent, boron 0.016 percent, yttrium 0.18 percent, iron0.26 percent and hafnium 0.15 percent, with the remainder essentiallycobalt except for impurities.

References Cited UNITED STATES PATENTS 3,005,705 10/1961 Cochardt --l713,202,506 8/1965 Deutsch 7517l 3,346,378 10/1967 Foster et al. 75l7lRICHARD O. DEAN, Primary Examiner

